I don't really consider a photo as "finished" so much as refined for particular kind of output.

I stage ratings, starting with one start for any image I find might be workable, then graduating up the ranking scale depending on how much I like the image and while doing so make edits that end with an image that can be used on the web. So roughly anything that is rated two or higher I know is ready for web use; though sometimes I may do final tweaks when looking to re-use an older image (and since ease of spot adjustments are one of Aperture's strengths, I think it makes sense to take advantage of that).

For other uses, such as print, I generally duplicate a "finished" version, then make additional changes needed for output - like color adjustments tailored to a profile, upsampling and final sharpening. The then encode all that into the version name with something like MYImage-360DPI-MPix-Sharp, and keep it stacked with the "final" version of the image.

Yes that feature is missing. It would be nice to have so you can't make accidental adjustments. I have just come up with this workaround: Export and reimport. Just make sure you export full resolution. Since the picture is finished, you don't need all the extra data from the raw, so you can just use high quality jpeg.

There is no way to protect a photo from further changes as of Aperture 3.0.3, but you can accomplish almost the same end result through a combination of features:

Open Preferences and go to the Advanced tab. Then check "Create new version when making adjustments".

Use meta data to indicate when you consider a photo done, then filter the browser on this meta data.

Use the vault feature to backup photo sets before and after you sort and edit the photos.

This way, you can easily remove accidental edits and revert to the version you consider the final one.

For example, I use the flag feature to mark photos for post processing (type "/" to toggle). In the photo browser view, I use the Flagged quick filter to only show flagged photos. When I'm done, I give the photo a star rating and remove the flag.

I can then find all the finished photos with smart albums that only show photos that have been edited and have a star rating.